THEORIES OF LAMARCK AND DARWIN 375 
and an opinion to which he held unwaveringly to the close 
of his life. It would be of great interest to determine when 
Lamarck changed his views, and upon what thisradical rever- 
sal of opinion “was based; but we have no sure record to 
depend upon. Since his theory is developed chiefly upon 
considerations of animal life, it is reasonable to assume that 
his evolutionary ideas took form in his mind after he began 
the serious study of animals. Doubtless, his mind having 
been prepared and his insight sharpened by his earlier studies, 
his observations in a new field supplied the data which led 
him directly to the conviction that species are unstable. 
As Packard, one of his recent biographers, points out, the 
first expression of his new views of which we have any record 
occurred in the spring of 1800, on the occasion of his opening 
lecture to his course on the invertebrates. This avowal of 
belief in the extensive alteration of species was published in 
1801 as the preface to his Sysléme des Animaux sans 
Vertébres. Here also he foreshadowed his theory of evo- 
lution, saying that nature, having formed the simplest 
organisms, “‘then with the aid of much time and favor- 
able circumstances . . . formed all the others.’’ It has 
been generally believed that Lamarck’s first public ex- 
pression of his views on evolution was published in 1802 
in his Recherches sur ?Organisation des Corps Vivans, 
but the researches of Packard and others have established 
the earlier date. 
Lamarck continued for several years to modify and am- 
plify the expression of his views. It is not necessary, how- 
ever, to follow the molding of his ideas on evolution as 
expressed in the opening lectures to his course in the years 
1800, 1802, 1803, and 1806, since we find them fully elab- 
orated in his Philosophie Zoologique, published in 1809, 
and this may be accepted as the standard source for the 
study of his theory. In this work he states two propositions 
ee 
